Surface vehicles are being developed to support long-range lunar or planetary exploration and the development of a lunar or planetary outpost. The surface vehicles will be heavier and will travel greater distances than the original Lunar Roving Vehicle (LRV) developed for the Apollo program in the late 1960s. New forms of tires are being developed to support much heavier weight loads. The tires are being developed to operate over many hundred times the travel distance as compared to those used on the Apollo LRV. Tires that have operational characteristics similar to passenger vehicles used on earth would benefit lunar and planetary surface missions and the like.
However, conventional rubber pneumatic tires cannot function acceptably on such surfaces and Space conditions. For example, rubber properties vary significantly between the cold temperatures experienced in shadow (down to 40 K) and the hot temperatures in sunlight (up to 400 K). Further, rubber degrades when exposed to direct solar radiation, without atmospheric protection. Finally, an air-filled tire is not permissible for manned lunar vehicles because of the possibility of deflation, e.g., a flat tire. To overcome these limitations, a tire design was developed for the Apollo LRV and was successfully used on Apollo missions 15, 16, and 17. This tire was woven from music wire, which was robust to lunar temperature variations and solar radiation, operated in vacuum, and did not require air for load support. This structure further functioned to contour to the lunar terrain, which facilitated traction and reduced vibration transfer to the Apollo LRV. However, because of increased weight and distance requirements for lunar vehicles, a tire with greater strength and durability is desirable.
The original wire mesh design of the Apollo LRV tire was not readily scalable. Specifically, the increase in wire diameter to create a tire that supported many times the load of the original design created two significant limitations: 1) the ability to contour to the terrain was lost, thus limiting traction and ability to isolate vibration; and 2) the increased wire stress limited functional life.
Limitations in the scalability of the original wire mesh constructions required alternate structural forms to be considered. Non-pneumatic tires based on helical spring geometries, i.e. a spring tire, lent additional design flexibility and control over performance requirements such as higher load carrying capability and improvements in traction (in particular, in soft soil) and obstacle envelopment performance.
However, designs based on helical spring constructions using conventional metals (e.g. Aluminum, Brass, Steel, and the like) can be limited when applied. The constructions exhibited a limited range over which they could be designed to function without undergoing permanent deformation during operation. This limitation occurred as a result of the helical geometries with conventional metals. As the load carrying capability of the tire is increased using the helical construction with conventional metals, a corresponding decrease in obstacle envelopment capability without damage (e.g. reversible deformation) is observed.
The aforementioned limitations with the helical based architectures result primarily from the use of conventional metals within the design. The conventional metals (e.g. aluminum, brass, steel, and the like) are what are commonly referred to as elastic-plastic materials. These materials can elastically deform (bond stretching that allows for reversible deformation) to about 0.3% strain in a material element point before an irreversible deformation mode, commonly referred to as plasticity, is initiated in the conventional metal. As a result of the limited amount of strain that the material can take prior to irreversibly deforming, structures such as helical springs (or other geometries that comprise deformations where a significant portion of the motion is not related to strain of the material) can be used in applications, in order to limit the amount of strain being imposed on the material during the deformation event.
Although, the utilization of structures like the spring will aid in allowing more deformation before the onset of plasticity, the utilization of the spring form with conventional metals can cause lower load carrying capabilities due to an increase in compliance. The helical structures with conventional metals results in tradeoffs due to the limitations of the conventional metal being used. Spring geometries designed with a specific pitch, wire diameter, and/or coil diameter for limiting and/or avoiding permanent deformation using conventional metals leads to more material being used. The additional material can result in an increase in metal volume of almost 500% in certain cases and leads to an increase in the overall mass of the tire, increased production cost, and a reduction in performance. A spring tire in accordance with the present invention overcomes these limitations, making the tire an innovative technological advance for Moon, Earth, and other planetary surfaces.